In recent years, the requirements for miniaturization and ever higher circuit density have made the surface characteristics of integrated circuit substrates (e.g., silicon wafers) and magnetic substrates (e.g., aluminum memory disks) increasingly more critical. The present state of the art for achieving the highest precision surface (a process generally referred to as "polishing") involves using a slurry usually made up of a mechanical abrasive in combination with a chemical reagent. This slurry is typically wiped over the face of a workpiece by a porous polishing pad. The pad must be firm enough to provide the necessary wiping action and porous enough to hold slurry. The most widely used materials for such polishing pads are taken from a class of materials known as poromerics. Poromerics are textile-like materials that usually contain a urethane-based impregnation or coating having a multitude of pores or cells.
Many of the popular poromeric materials used for polishing are similar to the material described in U.S. Pat. No. 3,284,274. These polishing poromerics are somewhat different from most other poromerics in that the surface of polishing poromerics contains large macropores or cells. It is believed that these larger cells act to hold slurry and thus aid in the polishing process. U.S. Pat. No. 3,504,457 describes the use of these materials in polishing silicon semiconductor substrates.
FIG. 1 shows an enlarged elevation view of a slice through a typical state-of-the-art polishing pad material. The top layer 10 is the cellular poromeric that comes into contact with the workpiece. The top layer contains cells 20 which may have a diameter anywhere from a few microns to several hundred microns, but typically between 50 and 150 microns. The walls 30 of cells 20 can be solid, but more typically the walls are made up of microporous sponge, the diameter of the micropores 35 being substantially less than 0.1, and more typically less than 0.01, times the diameter of cells 20. FIG. 2 is a further enlarged view of a portion of layer 10 showing the micropores 35 of walls 30 in relation to cells 20.
Because the top poromeric layer 10 tends to be mechanically fragile, it is mounted on a substrate 40 such as a plastic film, heavy paper or a woven or non-woven textile, sometimes by means of an adhesive 45. The most common substrate currently used is a non-woven felt that has been impregnated with a filler or binder to give it strength, dimensional stability and the required degree of cushioning or firmness.
To manufacture the poromeric layer 10 for a polishing material such as shown in FIG. 1, it is customary to coat a solution of polymer onto a substrate and then immerse the coated substrate into a bath that will cause coagulation of the polymer. Once the polymer has been fully coagulated, the remaining solvent is leached out and the product dried. FIG. 3 shows an enlarged elevation view of a slice through a resulting poromeric layer 10 with the cellular porous structure produced by the coagulation process. The poromeric layer 10 may be considered for purposes of discussion to comprise four layers, namely a top skin 50, an active open cell area 60, a cell bottom plane 70 and a base layer 80, mounted on a manufacturing substrate 85.
The next step in the manufacturing process is to remove the top skin 50 by passing the poromeric under a knife or, more commonly, under a rotating abrasive cylinder. Once the top skin 50 is removed, the underlying pores (active area 60) are exposed and open to the surface. The poromeric film 10 may then be used directly, or in some cases it may be stripped off its manufacturing substrate 85 and relaminated to a substrate 40 more suited to its end use. In either case, the product then resembles FIG. 1.
An important characteristic of this prior art product is the configuration of its cell structure. Because of the nature of the coagulation process, the pores and cells tend to increase in diameter as they penetrate deeper into the material. This means that the diameters of the cells 20 exposed on the working surface 15 of the material are relatively small compared to the underlying cell diameters. The large cells 22 in the interior of the poromeric layer are connected to the surface by a relatively small opening 24, much as a vase or Erlenmeyer flask has a large volume and small opening. It is generally believed that the large cell volume with respect to its opening size is important to the polishing process in order that the pad material may carry the maximum amount of slurry to the workpiece.
However, in the polishing process, this small opening 24 has several detrimental side effects. First, it makes the product short lived. Dross and spent slurry fill up the underlying cells and, because of the small opening to the surface, cannot easily be flushed out and replaced with fresh slurry. The dross becomes impacted in the cell and ultimately ends the cell's ability to carry slurry. Second, a relatively high percentage of the surface area at the working surface 15 is composed of cell walls. This undesirably causes a high wiping friction at the same time as it decreases the presentation of fresh slurry to the face of the workpiece.
Finally, at the end of the polishing cycle, the prior art material is difficult to clean and therefore slows down the termination of the polishing process. It is a customary final step in the polishing process to rinse the workpiece with pure water while the workpiece is still in the polishing environment. Because of the relatively small cell opening 24, it takes a long time to flush the slurry out of the cell and replace it with fresh water.
One of the potential solutions to the above mentioned problems is to make a cellular structure by conventional foam-making techniques. Foams made by incorporating air, blowing agents or microvoids all have cellular structures in which the cells are essentially spherical. The skin can be skived off such foams to produce a surface of open cells similar in appearance to the surface to the conventional polishing pads described above. In polishing performance, however, foam type products do not work well.
It is believed that foam's poor performance is due to the geometry of the cells. In a foam structure, the cells are formed by bubbles randomly dispersed throughout the structure. When the skin is sliced off, the bubbles are intersected and opened at random planes through the spheres. Some bubbles have only their very tops sheared off, others are sheared near their midpoints, and still others are sheared very near their bottoms. The resulting cellular surface contains cells that have either of two undesirable features. If the cell has been sheared above its midpoint, then the diameter of its opening is smaller than the diameter of the underlying cell, and the cell has the same difficult-to-clean properties of conventional polishing materials. If the cell has been sheared at or below its midpoint, then the depth of the cell is much less than its diameter and it will not hold enough slurry.